It is known that in mobile communications such as wireless LANS, the reception performance is degraded significantly by a fading phenomenon where the receiving electric field strength changes substantially due to reflection or scattering of radio waves. As reception techniques with which the influence of such a fading phenomenon is reduced, diversity reception methods are known for selecting a signal to demodulate from received signals obtained from a plurality of reception systems. The diversity reception methods include an antenna selection diversity reception method for selecting an antenna to receive a signal from a plurality of antennas.
Japanese Laid-Open Patent Publication No. 9-148973 discloses a conventional antenna selection diversity method. FIG. 10 is a block diagram showing a functional configuration of a conventional antenna selection diversity receiving apparatus 900 disclosed in Japanese Laid-Open Patent Publication No. 9-148973.
In the antenna selection diversity receiving apparatus 900 shown in FIG. 10, an antenna switching section 903 selects a signal from received signals received by first and second antennas 901 and 902, in response to an instruction from a control section 908. An AGC (Automatic Gain Control) circuit 904 controls a gain based on a control voltage from a control voltage generation section (hereinafter referred to as “VCO (Voltage Controlled Oscillator)”) 905 such that the output signal level of the received signal having been selected by the antenna switching section 903 is constant. The VCO 905 inputs a control voltage for controlling the gain of the AGC circuit 904 to the AGC circuit 904 based on an output signal from the AGC circuit 904. The control section 908 determines the power level of the received signal based on a control voltage from the VCO 905. For example, in the case where the signal is received by the first antenna 901, once the control section 908 determines the power level of the received signal, next the control section 908 allows the antenna switching section 903 to switch the antenna to the second antenna 902 so that a signal can be received by the second antenna 902. The control section 908 then determines the power level of the received signal received by the second antenna 902 based on a control voltage from the VCO 905. Thereafter, the control section 908 compares between the power level obtained upon reception by the first antenna 901 and the power level obtained upon reception by the second antenna 902, and instructs, based on the result of the comparison, the antenna switching section 903 to select an antenna having received a signal at an appropriate power level, from the first and second antennas 901 and 902. An A/D conversion section 906 performs an A/D conversion on the received signal. A demodulation circuit 907 demodulates the digital signal from the A/D conversion section 906. In this manner, the conventional antenna selection diversity receiving apparatus 900 selects an appropriate antenna according to changes in reception performance, thereby avoiding significant degradation of reception performance.
The above-described conventional configuration, however, is unable to accurately determine a received power unless a feedback loop made up of the AGC circuit 904 and the VCO 905 is converged. Therefore, in the control section 908, a power comparison requires as much time as it takes for the feedback loop to converge. In addition, in the case where there is a great difference in received power between the two antennas, the power to be inputted to the AGC circuit 904 changes abruptly at the moment of switching from one antenna to another, and thus convergence of the feedback loop requires more time.
In high-speed wireless packet communications such as wireless LANs, a plurality of terminals send wireless packets at a given time. The preamble length of a wireless packet is only several μsec. Thus, the receiving side needs to select an antenna within a preamble period of several μsec; however, in the conventional configuration such as that described above, since the AGC for a received-power comparison takes a long time, the selection of an antenna requires a long period of time and accordingly it is not possible to select an antenna within a preamble period of several μsec.